Regression in boost 1.82 from 1.79 - cannot add std::vector to json::object

[steve-numeus]

Version of Boost

1.82

Steps necessary to reproduce the problem

Upgrade from boost 1.79 to 1.82

Previously it was possible to add a std::vector<std::string> to a boost::json::obejct, this no longer works.

#include <iostream>
#include <vector>
#include <boost/json.hpp>

int main()
{
    std::vector<std::string> channels = {"foo", "bar"};
    boost::json::object params = {{"channels", std::move(channels)}};
    std::cout << params << '\n';
    return 0;
}

Error:

/usr/include/boost/json/impl/value_ref.hpp:49:12: error: no matching function for call to
    boost::json::value::value(std::remove_reference<std::vector<std::__cxx11::basic_string<char> >&>::type,
        std::remove_reference<boost::json::storage_ptr&>::type)

[grisumbras]

Duplicate of #823. In short, this was never supposed to work. Replace {{"channels", std::move(channels)}} with {{"channels", boost::json::value_from(channels)}}

[steve-numeus]

Thanks for the very quick update!

Since it did work before, (even if not supposed to) what is the reasoning not to support it?

Seems like a fairly ergonomic qol improvement to support?

[grisumbras]

The syntax was duplicitious. For example, you were trying to use a move there, which implies you expected some kind of move to happen. In reality, a conversion using value_from was performed. Now you need to use the conversion explicitly.

[steve-numeus]

Fair point. Thanks for the response

[steve-numeus]

Sorry for coming back to this, but I'm still struggling to understand the rationale here

If I've created a tag_invoke customisation point for my type:

inline void tag_invoke(boost::json::value_from_tag, boost::json::value& jv, Foo f)
{
    jv = boost::json::value_from(f.to_string(), jv.storage());
}

Why doesn't it participate in overload resolution when creating an object using an initializer list?

std::string s = "hello world";
Foo f = {};

boost::json::object o = {
    { "msg": s },
    { "foo": f }             // tag_invoke found
};

Now we're required to specify boost::json::value_from:

Foo f;
boost::json::object o = {
    { "msg": s },                          // implicit
    { "foo": boost::json::value_from(f) }  // tag_invoke no longer found
};

Rather unfortunately it results in arguably quite boilerplatey usage:

Before:

    return boost::json::object {
        {"time", time},
        {"inst", inst},
        {"ttm", ttm},
        {"domestic_rate", domestic_rate},
        {"foreign_rate", foreign_rate},
        {"spot_bid", spot_bid},
        {"spot_price", spot_price},
        {"spot_ask", spot_ask},
        {"underlying_price", underlying_price},
        {"usd_bid", usd_bid},
        {"usd_ask", usd_ask},
        {"iv_bid", iv_bid},
        {"iv_ask", iv_ask},
        {"iv_fit", iv_fit},
        {"iv_atmf_fit", iv_atmf_fit},
        {"log_moneyness", log_moneyness},
        {"log_moneyness_norm_fit", log_moneyness_norm_fit},
        {"tv_fit", tv_fit},
        {"delta_fit", delta_fit},
        {"gamma_fit", gamma_fit},
        {"theta_fit", theta_fit},
        {"vega_fit", vega_fit},
        {"vega_atmf_fit", vega_atmf_fit},
        {"rho_fit", rho_fit}
    };

After:

    return boost::json::object {
        {"time", boost::json::value_from(time)},
        {"inst", inst},
        {"ttm", boost::json::value_from(ttm)},
        {"domestic_rate", boost::json::value_from(domestic_rate)},
        {"foreign_rate", boost::json::value_from(foreign_rate)},
        {"spot_bid", boost::json::value_from(spot_bid)},
        {"spot_price", boost::json::value_from(spot_price)},
        {"spot_ask", boost::json::value_from(spot_ask)},
        {"underlying_price", boost::json::value_from(underlying_price)},
        {"usd_bid", boost::json::value_from(usd_bid)},
        {"usd_ask", boost::json::value_from(usd_ask)},
        {"iv_bid", boost::json::value_from(iv_bid)},
        {"iv_ask", boost::json::value_from(iv_ask)},
        {"iv_fit", boost::json::value_from(iv_fit)},
        {"iv_atmf_fit", boost::json::value_from(iv_atmf_fit)},
        {"log_moneyness", boost::json::value_from(log_moneyness)},
        {"log_moneyness_norm_fit", boost::json::value_from(log_moneyness_norm_fit)},
        {"tv_fit", boost::json::value_from(tv_fit)},
        {"delta_fit", boost::json::value_from(delta_fit)},
        {"gamma_fit", boost::json::value_from(gamma_fit)},
        {"theta_fit", boost::json::value_from(theta_fit)},
        {"vega_fit", boost::json::value_from(vega_fit)},
        {"vega_atmf_fit", boost::json::value_from(vega_atmf_fit)},
        {"rho_fit", boost::json::value_from(rho_fit)}
    };

[grisumbras]

tag_invoke overloads are meant for value_from and value_to. So the question is why would it be picked up by initializer_list assignment, rather than why wouldn't it be.

The boilerplate is unfortunate. I can recommend you using Boost.Describe interop instead of a custom tag_invoke overload.

[steve-numeus]

Ok, thanks for the tip, will look for boost.describe!

[grisumbras]

I was persuaded to bring back initializer_list construction using value_from under the hood. As per #1146 the requested functionality works again. Sorry for the inconvenience.